Pull-up and pull-down impedances are commonly used at an interface between two different types of logic devices, possibly operating at different power supply voltages. For example, in DDR2-SDRAM systems (DDR=Double Data Rate, SDRAM=Synchronous Dynamic Random Access Memory) center tapped driver pull-up and pull-down impedances in a driver circuit are usually kept equal or symmetric, as well as termination pull-up and pull-down impedances in a termination circuit. In case of a memory write operation, usually the memory controller or memory buffer makes use of a driver network to drive data towards a memory device and the memory device receives the data with a receiver or termination network. For a memory read operation the functionalities are vice versa. With that setup, a resulting common mode voltage at a receiving terminal fits to a target input reference voltage Vref, which is typically half the supply voltage of the receiving circuit.
In GDDR4-SGRAM systems (GDDR4=Graphics Double Data Rate (Version 4), SGRAM=Synchronous Graphics Random Access Memory) the setup of driver pull-up and pull-down impedances and termination impedances is implemented asymmetrically on the driver and asymmetrically at the receiver, however requiring equal supply voltage values for both, the driving circuit and the receiving circuit.
For a reduced power dissipation, it is desirable to further reduce the supply voltage on the memory controller or the memory buffer chip, like an advanced memory buffer (AMB), while keeping a higher supply voltage on the memory components.